1. Field of the Invention
The present invention relates to an inductance change detection circuit for detecting a change in the inductance of a coil. More specifically, the invention relates to an inductance change detection circuit for detecting a change in the position of an object based on a change in the inductance of a coil due to a change of a magnetic flux.
2. Description of the Related Art
A change in the position of an object that disrupts or indirectly changes the magnetic flux of a coil when the position of the object changes also results in a change in the inductance of the coil. It is therefore possible to detect a change in the object's position by detecting this inductance change as a DC signal, and devices used for this purpose are known from the literature. It is particularly desirable for such devices to detect this inductance change as a DC signal using a simple circuit design.
FIG. 8 shows a typical inductance change detection circuit according to the prior art as taught in Japanese Patent Laid-Open Publication No. 7-332910. FIG. 8A is a circuit diagram of this inductance change detection circuit, and FIG. 8B is a graph illustrating relationship between output pulses from a pulse power supply, and the output waveforms from a resistor R.sub.F in the inductance change detection circuit. Shown in FIG. 8A are a coil L of which the inductance changes according to the position of an object; a resistor R.sub.F having one end connected in series with the coil L and the other end to ground; and a pulse power supply 101.
The inductance change detection circuit shown in FIG. 8A detects the output from the output terminal, that is, the voltage between the resistor R.sub.F and ground, as an output voltage. When the position of the detected object has not changed from an initial position (referred to as the reference position below), this output voltage has a waveform as indicated by a solid line in FIG. 8B. When the position changes, however, the coil inductance L also changes, and the output voltage waveform changes as indicated by a dotted line in FIG. 8B. Because the output wave thus changes before and after the object changes position, it is possible to measure the voltage V.sub.0 and V.sub.01 at a specific time t.sub.k, and to detect the inductance change from this voltage difference.
A typical inductance change detection circuit of this type, however, has difficulty in detecting an inductance change with good precision when the signal amplitude from the output terminal drops or varies, however, because the change in coil inductance is detected from output voltage values detected at specific discrete points in time.
There is therefore a need for an inductance change detection circuit whereby a change in coil inductance can be detected with good precision using a simple circuit configuration.